Ultra high precision wavefront sensing for Extreme-AO on ELTs

High precision wavefront sensing is the key to detect low mass (potentially rocky) planets with
ELTs. Ideally, a wavefront sensor for an Extreme-AO system should be both very sensitive (to allow high speed wavefront correction) and very accurate (to allow precise calibration of residual starlight vs. planet light in the focal plane). I describe two options which meet these requirements: (1) non-linear curvature wavefront sensing is several orders of magnitude more sensitive than conventional WFSs, and can work at full sensitivity in open loop or in the visible (2) focal plane wavefront sensing combines high sensitivity and is free from non-common path errors. It can also measure light coherence, and therefore separate speckles from planets. Combining these two schemes is especially attractive for Extreme-AO systems aimed at direct imaging of exoplanets with ELTs. Laboratory demonstration of both Focal plane wavefront sensing is also be presented.